He worked as a Bevin Boy in the coal mining industry in Lanarkshire during the Second World War, before doing his National Service (1944–48).
Mabon was Chairman of the Glasgow University Labour Club (1948–50), then served as Chairman of the National Association of Labour Students in 1949–50, and finally as President of Glasgow University Union in 1951–52, and of the Scottish Union of Students, 1954–55.
In 1995, the competition was renamed the John Smith Memorial Mace and is now run by the English-Speaking Union.
He was political columnist for the Scottish Daily Record from 1955 to 1964, and studied under Henry Kissinger at Harvard University in 1963.
Although he supported Roy Jenkins at the Labour Party leadership election in 1976, James Callaghan appointed him as Minister of State in the Department of Energy (1976–79), where he took charge of North Sea oil.
Following Labour's defeat in the 1979 general election Mabon was tipped by The Glasgow Herald as the front-runner to succeed Bruce Millan as Shadow Secretary of State for Scotland, if the latter chose to move to another portfolio.
Mabon, whose first directorship had been at Radio Clyde in the 1970s, added a non-executive directorship with East Midlands Electricity to his place at Cairn; in 1992 he urged John Major's government to privatise British Coal in two halves with one going to an East Midland-led consortium including himself.
Mabon was a Fellow of the Royal Society of Arts (FRSA) and a Freeman of the City of London.